Cryptography (System.Security.Cryptography)

Cryptographically Secure Random Data

There are times when the framework's Random() class may not be considered random enough, given that it is based on a psuedo-random number generator. The framework's Crypto classes do, however, provide something more robust in the form of RNGCryptoServiceProvider.

The following code samples demonstrate how to generate Cryptographically Secure byte arrays, strings and numbers.

Random Byte Array

public static byte[] GenerateRandomData(int length)
{
    var rnd = new byte[length];
    using (var rng = new RNGCryptoServiceProvider())
        rng.GetBytes(rnd);
    return rnd;
}

Random Integer (with even distribution)

public static int GenerateRandomInt(int minVal=0, int maxVal=100)
{
    var rnd = new byte[4];
    using (var rng = new RNGCryptoServiceProvider())
        rng.GetBytes(rnd);
    var i = Math.Abs(BitConverter.ToInt32(rnd, 0));
    return Convert.ToInt32(i % (maxVal - minVal + 1) + minVal);
}

Random String

public static string GenerateRandomString(int length, string allowableChars=null)
{
    if (string.IsNullOrEmpty(allowableChars))
        allowableChars = @"ABCDEFGHIJKLMNOPQRSTUVWXYZ";

    // Generate random data
    var rnd = new byte[length];
    using (var rng = new RNGCryptoServiceProvider())
        rng.GetBytes(rnd);

    // Generate the output string
    var allowable = allowableChars.ToCharArray();
    var l = allowable.Length;
    var chars = new char[length];
    for (var i = 0; i < length; i++)
        chars[i] = allowable[rnd[i] % l];

    return new string(chars);
}

Fast Asymmetric File Encryption

Asymmetric encryption is often regarded as preferable to Symmetric encryption for transferring messages to other parties. This is mainly because it negates many of the risks related to the exchange of a shared key and ensures that whilst anyone with the public key can encrypt a message for the intended recipient, only that recipient can decrypt it. Unfortunately the major down-side of asymmetric encryption algorithms is that they are significantly slower than their symmetric cousins. As such the asymmetric encryption of files, especially large ones, can often be a very computationally intensive process.

In order to provide both security AND performance, a hybrid approach can be taken. This entails the cryptographically random generation of a key and initialization vector for Symmetric encryption. These values are then encrypted using an Asymmetric algorithm and written to the output file, before being used to encrypt the source data Symmetrically and appending it to the output.

This approach provides a high degree of both performance and security, in that the data is encrypted using a symmetric algorithm (fast) and the key and iv, both randomly generated (secure) are encrypted by an asymmetric algorithm (secure). It also has the added advantage that the same payload encrypted on different occasions will have very different cyphertext, because the symmetric keys are randomly generated each time.

The following class demonstrates asymmetric encryption of strings and byte arrays, as well as hybrid file encryption.

public static class AsymmetricProvider
{
    #region Key Generation
    public class KeyPair
    {
        public string PublicKey { get; set; }
        public string PrivateKey { get; set; }
    }

    public static KeyPair GenerateNewKeyPair(int keySize = 4096)
    {
        // KeySize is measured in bits. 1024 is the default, 2048 is better, 4096 is more robust but takes a fair bit longer to generate.
        using (var rsa = new RSACryptoServiceProvider(keySize))
        {
            return new KeyPair {PublicKey = rsa.ToXmlString(false), PrivateKey = rsa.ToXmlString(true)};
        }
    }

    #endregion

    #region Asymmetric Data Encryption and Decryption

    public static byte[] EncryptData(byte[] data, string publicKey)
    {
        using (var asymmetricProvider = new RSACryptoServiceProvider())
        {
            asymmetricProvider.FromXmlString(publicKey);
            return asymmetricProvider.Encrypt(data, true);
        }
    }

    public static byte[] DecryptData(byte[] data, string publicKey)
    {
        using (var asymmetricProvider = new RSACryptoServiceProvider())
        {
            asymmetricProvider.FromXmlString(publicKey);
            if (asymmetricProvider.PublicOnly)
                throw new Exception("The key provided is a public key and does not contain the private key elements required for decryption");
            return asymmetricProvider.Decrypt(data, true);
        }
    }

    public static string EncryptString(string value, string publicKey)
    {
        return Convert.ToBase64String(EncryptData(Encoding.UTF8.GetBytes(value), publicKey));
    }

    public static string DecryptString(string value, string privateKey)
    {
        return Encoding.UTF8.GetString(EncryptData(Convert.FromBase64String(value), privateKey));
    }

    #endregion

    #region Hybrid File Encryption and Decription

    public static void EncryptFile(string inputFilePath, string outputFilePath, string publicKey)
    {
        using (var symmetricCypher = new AesManaged())
        {
            // Generate random key and IV for symmetric encryption
            var key = new byte[symmetricCypher.KeySize / 8];
            var iv = new byte[symmetricCypher.BlockSize / 8];
            using (var rng = new RNGCryptoServiceProvider())
            {
                rng.GetBytes(key);
                rng.GetBytes(iv);
            }

            // Encrypt the symmetric key and IV
            var buf = new byte[key.Length + iv.Length];
            Array.Copy(key, buf, key.Length);
            Array.Copy(iv, 0, buf, key.Length, iv.Length);
            buf = EncryptData(buf, publicKey);

            var bufLen = BitConverter.GetBytes(buf.Length);

            // Symmetrically encrypt the data and write it to the file, along with the encrypted key and iv
            using (var cypherKey = symmetricCypher.CreateEncryptor(key, iv))
            using (var fsIn = new FileStream(inputFilePath, FileMode.Open))
            using (var fsOut = new FileStream(outputFilePath, FileMode.Create))
            using (var cs = new CryptoStream(fsOut, cypherKey, CryptoStreamMode.Write))
            {
                fsOut.Write(bufLen,0, bufLen.Length);
                fsOut.Write(buf, 0, buf.Length);
                fsIn.CopyTo(cs);
            }
        }
    }

    public static void DecryptFile(string inputFilePath, string outputFilePath, string privateKey)
    {
        using (var symmetricCypher = new AesManaged())
        using (var fsIn = new FileStream(inputFilePath, FileMode.Open))
        {
            // Determine the length of the encrypted key and IV
            var buf = new byte[sizeof(int)];
            fsIn.Read(buf, 0, buf.Length);
            var bufLen = BitConverter.ToInt32(buf, 0);

            // Read the encrypted key and IV data from the file and decrypt using the asymmetric algorithm
            buf = new byte[bufLen];
            fsIn.Read(buf, 0, buf.Length);
            buf = DecryptData(buf, privateKey);

            var key = new byte[symmetricCypher.KeySize / 8];
            var iv = new byte[symmetricCypher.BlockSize / 8];
            Array.Copy(buf, key, key.Length);
            Array.Copy(buf, key.Length, iv, 0, iv.Length);

            // Decript the file data using the symmetric algorithm
            using (var cypherKey = symmetricCypher.CreateDecryptor(key, iv))
            using (var fsOut = new FileStream(outputFilePath, FileMode.Create))
            using (var cs = new CryptoStream(fsOut, cypherKey, CryptoStreamMode.Write))
            {
                fsIn.CopyTo(cs);
            }
        }
    }

    #endregion

    #region Key Storage

    public static void WritePublicKey(string publicKeyFilePath, string publicKey)
    {
        File.WriteAllText(publicKeyFilePath, publicKey);
    }
    public static string ReadPublicKey(string publicKeyFilePath)
    {
        return File.ReadAllText(publicKeyFilePath);
    }

    private const string SymmetricSalt = "Stack_Overflow!"; // Change me!

    public static string ReadPrivateKey(string privateKeyFilePath, string password)
    {
        var salt = Encoding.UTF8.GetBytes(SymmetricSalt);
        var cypherText = File.ReadAllBytes(privateKeyFilePath);

        using (var cypher = new AesManaged())
        {
            var pdb = new Rfc2898DeriveBytes(password, salt);
            var key = pdb.GetBytes(cypher.KeySize / 8);
            var iv = pdb.GetBytes(cypher.BlockSize / 8);

            using (var decryptor = cypher.CreateDecryptor(key, iv))
            using (var msDecrypt = new MemoryStream(cypherText))
            using (var csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
            using (var srDecrypt = new StreamReader(csDecrypt))
            {
                return srDecrypt.ReadToEnd();
            }
        }
    }

    public static void WritePrivateKey(string privateKeyFilePath, string privateKey, string password)
    {
        var salt = Encoding.UTF8.GetBytes(SymmetricSalt);
        using (var cypher = new AesManaged())
        {
            var pdb = new Rfc2898DeriveBytes(password, salt);
            var key = pdb.GetBytes(cypher.KeySize / 8);
            var iv = pdb.GetBytes(cypher.BlockSize / 8);

            using (var encryptor = cypher.CreateEncryptor(key, iv))
            using (var fsEncrypt = new FileStream(privateKeyFilePath, FileMode.Create))
            using (var csEncrypt = new CryptoStream(fsEncrypt, encryptor, CryptoStreamMode.Write))
            using (var swEncrypt = new StreamWriter(csEncrypt))
            {
                swEncrypt.Write(privateKey);
            }
        }
    }

    #endregion
}

Example of use:

private static void HybridCryptoTest(string privateKeyPath, string privateKeyPassword, string inputPath)
{
    // Setup the test
    var publicKeyPath = Path.ChangeExtension(privateKeyPath, ".public");
    var outputPath = Path.Combine(Path.ChangeExtension(inputPath, ".enc"));
    var testPath = Path.Combine(Path.ChangeExtension(inputPath, ".test"));

    if (!File.Exists(privateKeyPath))
    {
        var keys = AsymmetricProvider.GenerateNewKeyPair(2048);
        AsymmetricProvider.WritePublicKey(publicKeyPath, keys.PublicKey);
        AsymmetricProvider.WritePrivateKey(privateKeyPath, keys.PrivateKey, privateKeyPassword);
    }

    // Encrypt the file
    var publicKey = AsymmetricProvider.ReadPublicKey(publicKeyPath);
    AsymmetricProvider.EncryptFile(inputPath, outputPath, publicKey);

    // Decrypt it again to compare against the source file
    var privateKey = AsymmetricProvider.ReadPrivateKey(privateKeyPath, privateKeyPassword);
    AsymmetricProvider.DecryptFile(outputPath, testPath, privateKey);

    // Check that the two files match
    var source = File.ReadAllBytes(inputPath);
    var dest = File.ReadAllBytes(testPath);

    if (source.Length != dest.Length)
        throw new Exception("Length does not match");

    if (source.Where((t, i) => t != dest[i]).Any())
        throw new Exception("Data mismatch");
}

Introduction to Symmetric and Asymmetric Encryption

You can improve the security for data transit or storing by implementing encrypting techniques. Basically there are two approaches when using System.Security.Cryptography: symmetric and asymmetric.


Symmetric Encryption

This method uses a private key in order to perform the data transformation.

Pros:

  • Symmetric algorithms consume less resources and are faster than asymmetric ones.
  • The amount of data you can encrypt is unlimited.

Cons:

  • Encryption and decryption use the same key. Someone will be able to decrypt your data if the key is compromised.
  • You could end up with many different secret keys to manage if you choose to use a different secret key for different data.

Under System.Security.Cryptography you have different classes that perform symmetric encryption, they are known as block ciphers:


Asymmetric Encryption

This method uses a combination of public and private keys in order to perform the data transformation.

Pros:

  • It uses larger keys than symmetric algorithms, thus they are less susceptible to being cracked by using brute force.
  • It is easier to guarantee who is able to encrypt and decrypt the data because it relies on two keys (public and private).

Cons:

  • There is a limit on the amount of data that you can encrypt. The limit is different for each algorithm and is typically proportional with the key size of the algorithm. For example, an RSACryptoServiceProvider object with a key length of 1,024 bits can only encrypt a message that is smaller than 128 bytes.
  • Asymmetric algorithms are very slow in comparison to symmetric algorithms.

Under System.Security.Cryptography you have access to different classes that perform asymmetric encryption:

Modern Examples of Symmetric Authenticated Encryption of a string

Cryptography is something very hard and after spending a lot of time reading different examples and seeing how easy it is to introduce some form of vulnerability I found an answer originally written by @jbtule that I think is very good. Enjoy reading:

"The general best practice for symmetric encryption is to use Authenticated Encryption with Associated Data (AEAD), however this isn't a part of the standard .net crypto libraries. So the first example uses AES256 and then HMAC256, a two step Encrypt then MAC, which requires more overhead and more keys.

The second example uses the simpler practice of AES256-GCM using the open source Bouncy Castle (via nuget).

Both examples have a main function that takes secret message string, key(s) and an optional non-secret payload and return and authenticated encrypted string optionally prepended with the non-secret data. Ideally you would use these with 256bit key(s) randomly generated see NewKey().

Both examples also have a helper methods that use a string password to generate the keys. These helper methods are provided as a convenience to match up with other examples, however they are far less secure because the strength of the password is going to be far weaker than a 256 bit key.

Update: Added byte[] overloads, and only the Gist has the full formatting with 4 spaces indent and api docs due to StackOverflow answer limits."


.NET Built-in Encrypt(AES)-Then-MAC(HMAC) [Gist]

/*
 * This work (Modern Encryption of a String C#, by James Tuley), 
 * identified by James Tuley, is free of known copyright restrictions.
 * https://gist.github.com/4336842
 * http://creativecommons.org/publicdomain/mark/1.0/ 
 */

using System;
using System.IO;
using System.Security.Cryptography;
using System.Text;

namespace Encryption
{
  public static class AESThenHMAC
  {
    private static readonly RandomNumberGenerator Random = RandomNumberGenerator.Create();
    
    //Preconfigured Encryption Parameters
    public static readonly int BlockBitSize = 128;
    public static readonly int KeyBitSize = 256;

    //Preconfigured Password Key Derivation Parameters
    public static readonly int SaltBitSize = 64;
    public static readonly int Iterations = 10000;
    public static readonly int MinPasswordLength = 12;

    /// <summary>
    /// Helper that generates a random key on each call.
    /// </summary>
    /// <returns></returns>
    public static byte[] NewKey()
    {
      var key = new byte[KeyBitSize / 8];
      Random.GetBytes(key);
      return key;
    }

    /// <summary>
    /// Simple Encryption (AES) then Authentication (HMAC) for a UTF8 Message.
    /// </summary>
    /// <param name="secretMessage">The secret message.</param>
    /// <param name="cryptKey">The crypt key.</param>
    /// <param name="authKey">The auth key.</param>
    /// <param name="nonSecretPayload">(Optional) Non-Secret Payload.</param>
    /// <returns>
    /// Encrypted Message
    /// </returns>
    /// <exception cref="System.ArgumentException">Secret Message Required!;secretMessage</exception>
    /// <remarks>
    /// Adds overhead of (Optional-Payload + BlockSize(16) + Message-Padded-To-Blocksize +  HMac-Tag(32)) * 1.33 Base64
    /// </remarks>
    public static string SimpleEncrypt(string secretMessage, byte[] cryptKey, byte[] authKey,
                       byte[] nonSecretPayload = null)
    {
      if (string.IsNullOrEmpty(secretMessage))
        throw new ArgumentException("Secret Message Required!", "secretMessage");

      var plainText = Encoding.UTF8.GetBytes(secretMessage);
      var cipherText = SimpleEncrypt(plainText, cryptKey, authKey, nonSecretPayload);
      return Convert.ToBase64String(cipherText);
    }

    /// <summary>
    /// Simple Authentication (HMAC) then Decryption (AES) for a secrets UTF8 Message.
    /// </summary>
    /// <param name="encryptedMessage">The encrypted message.</param>
    /// <param name="cryptKey">The crypt key.</param>
    /// <param name="authKey">The auth key.</param>
    /// <param name="nonSecretPayloadLength">Length of the non secret payload.</param>
    /// <returns>
    /// Decrypted Message
    /// </returns>
    /// <exception cref="System.ArgumentException">Encrypted Message Required!;encryptedMessage</exception>
    public static string SimpleDecrypt(string encryptedMessage, byte[] cryptKey, byte[] authKey,
                       int nonSecretPayloadLength = 0)
    {
      if (string.IsNullOrWhiteSpace(encryptedMessage))
        throw new ArgumentException("Encrypted Message Required!", "encryptedMessage");

      var cipherText = Convert.FromBase64String(encryptedMessage);
      var plainText = SimpleDecrypt(cipherText, cryptKey, authKey, nonSecretPayloadLength);
      return plainText == null ? null : Encoding.UTF8.GetString(plainText);
    }

    /// <summary>
    /// Simple Encryption (AES) then Authentication (HMAC) of a UTF8 message
    /// using Keys derived from a Password (PBKDF2).
    /// </summary>
    /// <param name="secretMessage">The secret message.</param>
    /// <param name="password">The password.</param>
    /// <param name="nonSecretPayload">The non secret payload.</param>
    /// <returns>
    /// Encrypted Message
    /// </returns>
    /// <exception cref="System.ArgumentException">password</exception>
    /// <remarks>
    /// Significantly less secure than using random binary keys.
    /// Adds additional non secret payload for key generation parameters.
    /// </remarks>
    public static string SimpleEncryptWithPassword(string secretMessage, string password,
                             byte[] nonSecretPayload = null)
    {
      if (string.IsNullOrEmpty(secretMessage))
        throw new ArgumentException("Secret Message Required!", "secretMessage");

      var plainText = Encoding.UTF8.GetBytes(secretMessage);
      var cipherText = SimpleEncryptWithPassword(plainText, password, nonSecretPayload);
      return Convert.ToBase64String(cipherText);
    }

    /// <summary>
    /// Simple Authentication (HMAC) and then Descryption (AES) of a UTF8 Message
    /// using keys derived from a password (PBKDF2). 
    /// </summary>
    /// <param name="encryptedMessage">The encrypted message.</param>
    /// <param name="password">The password.</param>
    /// <param name="nonSecretPayloadLength">Length of the non secret payload.</param>
    /// <returns>
    /// Decrypted Message
    /// </returns>
    /// <exception cref="System.ArgumentException">Encrypted Message Required!;encryptedMessage</exception>
    /// <remarks>
    /// Significantly less secure than using random binary keys.
    /// </remarks>
    public static string SimpleDecryptWithPassword(string encryptedMessage, string password,
                             int nonSecretPayloadLength = 0)
    {
      if (string.IsNullOrWhiteSpace(encryptedMessage))
        throw new ArgumentException("Encrypted Message Required!", "encryptedMessage");

      var cipherText = Convert.FromBase64String(encryptedMessage);
      var plainText = SimpleDecryptWithPassword(cipherText, password, nonSecretPayloadLength);
      return plainText == null ? null : Encoding.UTF8.GetString(plainText);
    }

    public static byte[] SimpleEncrypt(byte[] secretMessage, byte[] cryptKey, byte[] authKey, byte[] nonSecretPayload = null)
    {
      //User Error Checks
      if (cryptKey == null || cryptKey.Length != KeyBitSize / 8)
        throw new ArgumentException(String.Format("Key needs to be {0} bit!", KeyBitSize), "cryptKey");

      if (authKey == null || authKey.Length != KeyBitSize / 8)
        throw new ArgumentException(String.Format("Key needs to be {0} bit!", KeyBitSize), "authKey");

      if (secretMessage == null || secretMessage.Length < 1)
        throw new ArgumentException("Secret Message Required!", "secretMessage");

      //non-secret payload optional
      nonSecretPayload = nonSecretPayload ?? new byte[] { };

      byte[] cipherText;
      byte[] iv;

      using (var aes = new AesManaged
      {
        KeySize = KeyBitSize,
        BlockSize = BlockBitSize,
        Mode = CipherMode.CBC,
        Padding = PaddingMode.PKCS7
      })
      {

        //Use random IV
        aes.GenerateIV();
        iv = aes.IV;

        using (var encrypter = aes.CreateEncryptor(cryptKey, iv))
        using (var cipherStream = new MemoryStream())
        {
          using (var cryptoStream = new CryptoStream(cipherStream, encrypter, CryptoStreamMode.Write))
          using (var binaryWriter = new BinaryWriter(cryptoStream))
          {
            //Encrypt Data
            binaryWriter.Write(secretMessage);
          }

          cipherText = cipherStream.ToArray();
        }

      }

      //Assemble encrypted message and add authentication
      using (var hmac = new HMACSHA256(authKey))
      using (var encryptedStream = new MemoryStream())
      {
        using (var binaryWriter = new BinaryWriter(encryptedStream))
        {
          //Prepend non-secret payload if any
          binaryWriter.Write(nonSecretPayload);
          //Prepend IV
          binaryWriter.Write(iv);
          //Write Ciphertext
          binaryWriter.Write(cipherText);
          binaryWriter.Flush();

          //Authenticate all data
          var tag = hmac.ComputeHash(encryptedStream.ToArray());
          //Postpend tag
          binaryWriter.Write(tag);
        }
        return encryptedStream.ToArray();
      }

    }

    public static byte[] SimpleDecrypt(byte[] encryptedMessage, byte[] cryptKey, byte[] authKey, int nonSecretPayloadLength = 0)
    {

      //Basic Usage Error Checks
      if (cryptKey == null || cryptKey.Length != KeyBitSize / 8)
        throw new ArgumentException(String.Format("CryptKey needs to be {0} bit!", KeyBitSize), "cryptKey");

      if (authKey == null || authKey.Length != KeyBitSize / 8)
        throw new ArgumentException(String.Format("AuthKey needs to be {0} bit!", KeyBitSize), "authKey");

      if (encryptedMessage == null || encryptedMessage.Length == 0)
        throw new ArgumentException("Encrypted Message Required!", "encryptedMessage");

      using (var hmac = new HMACSHA256(authKey))
      {
        var sentTag = new byte[hmac.HashSize / 8];
        //Calculate Tag
        var calcTag = hmac.ComputeHash(encryptedMessage, 0, encryptedMessage.Length - sentTag.Length);
        var ivLength = (BlockBitSize / 8);

        //if message length is to small just return null
        if (encryptedMessage.Length < sentTag.Length + nonSecretPayloadLength + ivLength)
          return null;

        //Grab Sent Tag
        Array.Copy(encryptedMessage, encryptedMessage.Length - sentTag.Length, sentTag, 0, sentTag.Length);

        //Compare Tag with constant time comparison
        var compare = 0;
        for (var i = 0; i < sentTag.Length; i++)
          compare |= sentTag[i] ^ calcTag[i]; 

        //if message doesn't authenticate return null
        if (compare != 0)
          return null;

        using (var aes = new AesManaged
        {
          KeySize = KeyBitSize,
          BlockSize = BlockBitSize,
          Mode = CipherMode.CBC,
          Padding = PaddingMode.PKCS7
        })
        {

          //Grab IV from message
          var iv = new byte[ivLength];
          Array.Copy(encryptedMessage, nonSecretPayloadLength, iv, 0, iv.Length);

          using (var decrypter = aes.CreateDecryptor(cryptKey, iv))
          using (var plainTextStream = new MemoryStream())
          {
            using (var decrypterStream = new CryptoStream(plainTextStream, decrypter, CryptoStreamMode.Write))
            using (var binaryWriter = new BinaryWriter(decrypterStream))
            {
              //Decrypt Cipher Text from Message
              binaryWriter.Write(
                encryptedMessage,
                nonSecretPayloadLength + iv.Length,
                encryptedMessage.Length - nonSecretPayloadLength - iv.Length - sentTag.Length
              );
            }
            //Return Plain Text
            return plainTextStream.ToArray();
          }
        }
      }
    }

    public static byte[] SimpleEncryptWithPassword(byte[] secretMessage, string password, byte[] nonSecretPayload = null)
    {
      nonSecretPayload = nonSecretPayload ?? new byte[] {};

      //User Error Checks
      if (string.IsNullOrWhiteSpace(password) || password.Length < MinPasswordLength)
        throw new ArgumentException(String.Format("Must have a password of at least {0} characters!", MinPasswordLength), "password");

      if (secretMessage == null || secretMessage.Length ==0)
        throw new ArgumentException("Secret Message Required!", "secretMessage");

      var payload = new byte[((SaltBitSize / 8) * 2) + nonSecretPayload.Length];

      Array.Copy(nonSecretPayload, payload, nonSecretPayload.Length);
      int payloadIndex = nonSecretPayload.Length;

      byte[] cryptKey;
      byte[] authKey;
      //Use Random Salt to prevent pre-generated weak password attacks.
      using (var generator = new Rfc2898DeriveBytes(password, SaltBitSize / 8, Iterations))
      {
        var salt = generator.Salt;

        //Generate Keys
        cryptKey = generator.GetBytes(KeyBitSize / 8);

        //Create Non Secret Payload
        Array.Copy(salt, 0, payload, payloadIndex, salt.Length);
        payloadIndex += salt.Length;
      }

      //Deriving separate key, might be less efficient than using HKDF, 
      //but now compatible with RNEncryptor which had a very similar wireformat and requires less code than HKDF.
      using (var generator = new Rfc2898DeriveBytes(password, SaltBitSize / 8, Iterations))
      {
        var salt = generator.Salt;

        //Generate Keys
        authKey = generator.GetBytes(KeyBitSize / 8);

        //Create Rest of Non Secret Payload
        Array.Copy(salt, 0, payload, payloadIndex, salt.Length);
      }

      return SimpleEncrypt(secretMessage, cryptKey, authKey, payload);
    }

    public static byte[] SimpleDecryptWithPassword(byte[] encryptedMessage, string password, int nonSecretPayloadLength = 0)
    {
      //User Error Checks
      if (string.IsNullOrWhiteSpace(password) || password.Length < MinPasswordLength)
        throw new ArgumentException(String.Format("Must have a password of at least {0} characters!", MinPasswordLength), "password");

      if (encryptedMessage == null || encryptedMessage.Length == 0)
        throw new ArgumentException("Encrypted Message Required!", "encryptedMessage");

      var cryptSalt = new byte[SaltBitSize / 8];
      var authSalt = new byte[SaltBitSize / 8];

      //Grab Salt from Non-Secret Payload
      Array.Copy(encryptedMessage, nonSecretPayloadLength, cryptSalt, 0, cryptSalt.Length);
      Array.Copy(encryptedMessage, nonSecretPayloadLength + cryptSalt.Length, authSalt, 0, authSalt.Length);

      byte[] cryptKey;
      byte[] authKey;

      //Generate crypt key
      using (var generator = new Rfc2898DeriveBytes(password, cryptSalt, Iterations))
      {
        cryptKey = generator.GetBytes(KeyBitSize / 8);
      }
      //Generate auth key
      using (var generator = new Rfc2898DeriveBytes(password, authSalt, Iterations))
      {
        authKey = generator.GetBytes(KeyBitSize / 8);
      }

      return SimpleDecrypt(encryptedMessage, cryptKey, authKey, cryptSalt.Length + authSalt.Length + nonSecretPayloadLength);
    }
  }
}

Bouncy Castle AES-GCM [Gist]

/*
 * This work (Modern Encryption of a String C#, by James Tuley), 
 * identified by James Tuley, is free of known copyright restrictions.
 * https://gist.github.com/4336842
 * http://creativecommons.org/publicdomain/mark/1.0/ 
 */

using System;
using System.IO;
using System.Text;
using Org.BouncyCastle.Crypto;
using Org.BouncyCastle.Crypto.Engines;
using Org.BouncyCastle.Crypto.Generators;
using Org.BouncyCastle.Crypto.Modes;
using Org.BouncyCastle.Crypto.Parameters;
using Org.BouncyCastle.Security;
namespace Encryption
{

  public static class AESGCM
  {
    private static readonly SecureRandom Random = new SecureRandom();

    //Preconfigured Encryption Parameters
    public static readonly int NonceBitSize = 128;
    public static readonly int MacBitSize = 128;
    public static readonly int KeyBitSize = 256;

    //Preconfigured Password Key Derivation Parameters
    public static readonly int SaltBitSize = 128;
    public static readonly int Iterations = 10000;
    public static readonly int MinPasswordLength = 12;


    /// <summary>
    /// Helper that generates a random new key on each call.
    /// </summary>
    /// <returns></returns>
    public static byte[] NewKey()
    {
      var key = new byte[KeyBitSize / 8];
      Random.NextBytes(key);
      return key;
    }

    /// <summary>
    /// Simple Encryption And Authentication (AES-GCM) of a UTF8 string.
    /// </summary>
    /// <param name="secretMessage">The secret message.</param>
    /// <param name="key">The key.</param>
    /// <param name="nonSecretPayload">Optional non-secret payload.</param>
    /// <returns>
    /// Encrypted Message
    /// </returns>
    /// <exception cref="System.ArgumentException">Secret Message Required!;secretMessage</exception>
    /// <remarks>
    /// Adds overhead of (Optional-Payload + BlockSize(16) + Message +  HMac-Tag(16)) * 1.33 Base64
    /// </remarks>
    public static string SimpleEncrypt(string secretMessage, byte[] key, byte[] nonSecretPayload = null)
    {
      if (string.IsNullOrEmpty(secretMessage))
        throw new ArgumentException("Secret Message Required!", "secretMessage");

      var plainText = Encoding.UTF8.GetBytes(secretMessage);
      var cipherText = SimpleEncrypt(plainText, key, nonSecretPayload);
      return Convert.ToBase64String(cipherText);
    }


    /// <summary>
    /// Simple Decryption & Authentication (AES-GCM) of a UTF8 Message
    /// </summary>
    /// <param name="encryptedMessage">The encrypted message.</param>
    /// <param name="key">The key.</param>
    /// <param name="nonSecretPayloadLength">Length of the optional non-secret payload.</param>
    /// <returns>Decrypted Message</returns>
    public static string SimpleDecrypt(string encryptedMessage, byte[] key, int nonSecretPayloadLength = 0)
    {
      if (string.IsNullOrEmpty(encryptedMessage))
        throw new ArgumentException("Encrypted Message Required!", "encryptedMessage");

      var cipherText = Convert.FromBase64String(encryptedMessage);
      var plainText = SimpleDecrypt(cipherText, key, nonSecretPayloadLength);
      return plainText == null ? null : Encoding.UTF8.GetString(plainText);
    }

    /// <summary>
    /// Simple Encryption And Authentication (AES-GCM) of a UTF8 String
    /// using key derived from a password (PBKDF2).
    /// </summary>
    /// <param name="secretMessage">The secret message.</param>
    /// <param name="password">The password.</param>
    /// <param name="nonSecretPayload">The non secret payload.</param>
    /// <returns>
    /// Encrypted Message
    /// </returns>
    /// <remarks>
    /// Significantly less secure than using random binary keys.
    /// Adds additional non secret payload for key generation parameters.
    /// </remarks>
    public static string SimpleEncryptWithPassword(string secretMessage, string password,
                             byte[] nonSecretPayload = null)
    {
      if (string.IsNullOrEmpty(secretMessage))
        throw new ArgumentException("Secret Message Required!", "secretMessage");

      var plainText = Encoding.UTF8.GetBytes(secretMessage);
      var cipherText = SimpleEncryptWithPassword(plainText, password, nonSecretPayload);
      return Convert.ToBase64String(cipherText);
    }


    /// <summary>
    /// Simple Decryption and Authentication (AES-GCM) of a UTF8 message
    /// using a key derived from a password (PBKDF2)
    /// </summary>
    /// <param name="encryptedMessage">The encrypted message.</param>
    /// <param name="password">The password.</param>
    /// <param name="nonSecretPayloadLength">Length of the non secret payload.</param>
    /// <returns>
    /// Decrypted Message
    /// </returns>
    /// <exception cref="System.ArgumentException">Encrypted Message Required!;encryptedMessage</exception>
    /// <remarks>
    /// Significantly less secure than using random binary keys.
    /// </remarks>
    public static string SimpleDecryptWithPassword(string encryptedMessage, string password,
                             int nonSecretPayloadLength = 0)
    {
      if (string.IsNullOrWhiteSpace(encryptedMessage))
        throw new ArgumentException("Encrypted Message Required!", "encryptedMessage");

      var cipherText = Convert.FromBase64String(encryptedMessage);
      var plainText = SimpleDecryptWithPassword(cipherText, password, nonSecretPayloadLength);
      return plainText == null ? null : Encoding.UTF8.GetString(plainText);
    }

    public static byte[] SimpleEncrypt(byte[] secretMessage, byte[] key, byte[] nonSecretPayload = null)
    {
      //User Error Checks
      if (key == null || key.Length != KeyBitSize / 8)
        throw new ArgumentException(String.Format("Key needs to be {0} bit!", KeyBitSize), "key");

      if (secretMessage == null || secretMessage.Length == 0)
        throw new ArgumentException("Secret Message Required!", "secretMessage");

      //Non-secret Payload Optional
      nonSecretPayload = nonSecretPayload ?? new byte[] { };

      //Using random nonce large enough not to repeat
      var nonce = new byte[NonceBitSize / 8];
      Random.NextBytes(nonce, 0, nonce.Length);

      var cipher = new GcmBlockCipher(new AesFastEngine());
      var parameters = new AeadParameters(new KeyParameter(key), MacBitSize, nonce, nonSecretPayload);
      cipher.Init(true, parameters);

      //Generate Cipher Text With Auth Tag
      var cipherText = new byte[cipher.GetOutputSize(secretMessage.Length)];
      var len = cipher.ProcessBytes(secretMessage, 0, secretMessage.Length, cipherText, 0);
      cipher.DoFinal(cipherText, len);

      //Assemble Message
      using (var combinedStream = new MemoryStream())
      {
        using (var binaryWriter = new BinaryWriter(combinedStream))
        {
          //Prepend Authenticated Payload
          binaryWriter.Write(nonSecretPayload);
          //Prepend Nonce
          binaryWriter.Write(nonce);
          //Write Cipher Text
          binaryWriter.Write(cipherText);
        }
        return combinedStream.ToArray();
      }
    }

    public static byte[] SimpleDecrypt(byte[] encryptedMessage, byte[] key, int nonSecretPayloadLength = 0)
    {
      //User Error Checks
      if (key == null || key.Length != KeyBitSize / 8)
        throw new ArgumentException(String.Format("Key needs to be {0} bit!", KeyBitSize), "key");

      if (encryptedMessage == null || encryptedMessage.Length == 0)
        throw new ArgumentException("Encrypted Message Required!", "encryptedMessage");

      using (var cipherStream = new MemoryStream(encryptedMessage))
      using (var cipherReader = new BinaryReader(cipherStream))
      {
        //Grab Payload
        var nonSecretPayload = cipherReader.ReadBytes(nonSecretPayloadLength);

        //Grab Nonce
        var nonce = cipherReader.ReadBytes(NonceBitSize / 8);
       
        var cipher = new GcmBlockCipher(new AesFastEngine());
        var parameters = new AeadParameters(new KeyParameter(key), MacBitSize, nonce, nonSecretPayload);
        cipher.Init(false, parameters);

        //Decrypt Cipher Text
        var cipherText = cipherReader.ReadBytes(encryptedMessage.Length - nonSecretPayloadLength - nonce.Length);
        var plainText = new byte[cipher.GetOutputSize(cipherText.Length)];  

        try
        {
          var len = cipher.ProcessBytes(cipherText, 0, cipherText.Length, plainText, 0);
          cipher.DoFinal(plainText, len);

        }
        catch (InvalidCipherTextException)
        {
          //Return null if it doesn't authenticate
          return null;
        }

        return plainText;
      }

    }

    public static byte[] SimpleEncryptWithPassword(byte[] secretMessage, string password, byte[] nonSecretPayload = null)
    {
      nonSecretPayload = nonSecretPayload ?? new byte[] {};

      //User Error Checks
      if (string.IsNullOrWhiteSpace(password) || password.Length < MinPasswordLength)
        throw new ArgumentException(String.Format("Must have a password of at least {0} characters!", MinPasswordLength), "password");

      if (secretMessage == null || secretMessage.Length == 0)
        throw new ArgumentException("Secret Message Required!", "secretMessage");

      var generator = new Pkcs5S2ParametersGenerator();

      //Use Random Salt to minimize pre-generated weak password attacks.
      var salt = new byte[SaltBitSize / 8];
      Random.NextBytes(salt);

      generator.Init(
        PbeParametersGenerator.Pkcs5PasswordToBytes(password.ToCharArray()),
        salt,
        Iterations);

      //Generate Key
      var key = (KeyParameter)generator.GenerateDerivedMacParameters(KeyBitSize);

      //Create Full Non Secret Payload
      var payload = new byte[salt.Length + nonSecretPayload.Length];
      Array.Copy(nonSecretPayload, payload, nonSecretPayload.Length);
      Array.Copy(salt,0, payload,nonSecretPayload.Length, salt.Length);

      return SimpleEncrypt(secretMessage, key.GetKey(), payload);
    }

    public static byte[] SimpleDecryptWithPassword(byte[] encryptedMessage, string password, int nonSecretPayloadLength = 0)
    {
      //User Error Checks
      if (string.IsNullOrWhiteSpace(password) || password.Length < MinPasswordLength)
        throw new ArgumentException(String.Format("Must have a password of at least {0} characters!", MinPasswordLength), "password");

      if (encryptedMessage == null || encryptedMessage.Length == 0)
        throw new ArgumentException("Encrypted Message Required!", "encryptedMessage");

      var generator = new Pkcs5S2ParametersGenerator();

      //Grab Salt from Payload
      var salt = new byte[SaltBitSize / 8];
      Array.Copy(encryptedMessage, nonSecretPayloadLength, salt, 0, salt.Length);

      generator.Init(
        PbeParametersGenerator.Pkcs5PasswordToBytes(password.ToCharArray()),
        salt,
        Iterations);

      //Generate Key
      var key = (KeyParameter)generator.GenerateDerivedMacParameters(KeyBitSize);

      return SimpleDecrypt(encryptedMessage, key.GetKey(), salt.Length + nonSecretPayloadLength);
    }
  }
}

Password Hashing

Passwords should never be stored as plain text! They should be hashed with a randomly generated salt (to defend against rainbow table attacks) using a slow password hashing algorithm. A high number of iterations (> 10k) can be used to slow down brute force attacks. A delay of ~100ms is acceptable to a user logging in, but makes breaking a long password difficult. When choosing a number of iterations you should use the maximum tolerable value for your application and increase it as computer performance improves. You will also need to consider stopping repeated requests which could be used as a DoS attack.

When hashing for the first time a salt can be generated for you, the resulting hash and salt can then be stored to a file.

private void firstHash(string userName, string userPassword, int numberOfItterations)
{
    Rfc2898DeriveBytes PBKDF2 = new Rfc2898DeriveBytes(userPassword, 8, numberOfItterations);    //Hash the password with a 8 byte salt
    byte[] hashedPassword = PBKDF2.GetBytes(20);    //Returns a 20 byte hash
    byte[] salt = PBKDF2.Salt;
    writeHashToFile(userName, hashedPassword, salt, numberOfItterations); //Store the hashed password with the salt and number of itterations to check against future password entries
}

Checking an existing users password, read their hash and salt from a file and compare to the hash of the entered password

private bool checkPassword(string userName, string userPassword, int numberOfItterations)
{
    byte[] usersHash = getUserHashFromFile(userName);
    byte[] userSalt = getUserSaltFromFile(userName);
    Rfc2898DeriveBytes PBKDF2 = new Rfc2898DeriveBytes(userPassword, userSalt, numberOfItterations);    //Hash the password with the users salt
    byte[] hashedPassword = PBKDF2.GetBytes(20);    //Returns a 20 byte hash            
    bool passwordsMach = comparePasswords(usersHash, hashedPassword);    //Compares byte arrays
    return passwordsMach;
}

Simple Symmetric File Encryption

The following code sample demonstrates a quick and easy means of encrypting and decrypting files using the AES symmetric encryption algorithm.

The code randomly generates the Salt and Initialization Vectors each time a file is encrypted, meaning that encrypting the same file with the same password will always lead to different output. The salt and IV are written to the output file so that only the password is required to decrypt it.

public static void ProcessFile(string inputPath, string password, bool encryptMode, string outputPath)
{
    using (var cypher = new AesManaged())
    using (var fsIn = new FileStream(inputPath, FileMode.Open))
    using (var fsOut = new FileStream(outputPath, FileMode.Create))
    {
        const int saltLength = 256;
        var salt = new byte[saltLength];
        var iv = new byte[cypher.BlockSize / 8];

        if (encryptMode)
        {
            // Generate random salt and IV, then write them to file
            using (var rng = new RNGCryptoServiceProvider())
            {
                rng.GetBytes(salt);
                rng.GetBytes(iv);
            }
            fsOut.Write(salt, 0, salt.Length);
            fsOut.Write(iv, 0, iv.Length);
        }
        else
        {
            // Read the salt and IV from the file
            fsIn.Read(salt, 0, saltLength);
            fsIn.Read(iv, 0, iv.Length);
        }

        // Generate a secure password, based on the password and salt provided
        var pdb = new Rfc2898DeriveBytes(password, salt);
        var key = pdb.GetBytes(cypher.KeySize / 8);

        // Encrypt or decrypt the file
        using (var cryptoTransform = encryptMode
            ? cypher.CreateEncryptor(key, iv)
            : cypher.CreateDecryptor(key, iv))
        using (var cs = new CryptoStream(fsOut, cryptoTransform, CryptoStreamMode.Write))
        {
            fsIn.CopyTo(cs);
        }
    }
}